The present invention relates to a method and apparatus for controlling light emission of a flash, and more particularly, to a method and apparatus for controlling light emission of a flash in an image sensor, e.g., a complementary metal oxide semiconductor (CMOS) sensor, that uses a rolling shutter.
In general, digital cameras include a flash using a Xenon discharge tube. Flashes adjust flash intensity to illuminate a subject when there is not enough available light to adequately expose an image sensor. Such adjustment depends on a flash emission time. That is, the longer the flash emission time, the brighter the subject, and the shorter the flash emission time, the darker the subject. Since the brightness of the subject is controlled by the flash emission time, an algorithm for controlling the flash emission time is required and the following two algorithms are often used.
In an algorithm based on an optical sensor method using an optical sensor, a flash emits light and, at the same time, the brightness of a subject is measured. When the brightness of the subject reaches a preset brightness level due to the light emitted by the flash, the flash stops emitting the light. The subject is photographed for a flash emission time of the flash.
In an algorithm based on a pre-emission method, a flash emits light at a low brightness, the brightness of a subject is measured, a flash emission time of the flash taken to make the subject reach a preset brightness level is calculated, and the flash emits light for the calculated flash emission time to photograph the subject.
A complementary metal oxide semiconductor (CMOS) image sensor operates by using a rolling shutter in a live view mode. If the CMOS image sensor operates by using the rolling shutter and a subject or a photographer moves during shooting, an image distortion due to an exposure time difference, which is known as a rolling shutter effect, may occur.
There are the following problems in applying the aforesaid algorithms based on the two methods to the CMOS image sensor. The optical sensor method can be performed in the same manner as a conventional method, but since the optical sensor is used, the price of the optical sensor is added, it is difficult to correct a partial exposure of the subject, and a process of adjusting the optical sensor has to be added to a production process.
Although the pre-emission method is widely used, if the pre-emission method is applied to the CMOS image sensor that operates by using the rolling shutter, a partial exposure occurs. That is, if the brightness of an image is locally measured during a pre-emission, the brightness cannot be normally calculated. Such a partial exposure commonly occurs in image sensors that operate by using rolling shutters.